Expose C++ obj pointers to Python API #595

Job	Run time
Build GHEX and run unit tests (cpu-gcc-mpi-atlas-parmetis, ghcr.io/gridtools/gridtools-base:gcc-9...	3m 5s
Build GHEX and run unit tests (cpu-gcc-ucx-atlas-parmetis, ghcr.io/gridtools/gridtools-base:gcc-9...	3m 5s
Build GHEX and run unit tests (cpu-clang-mpi-atlas-parmetis, ghcr.io/gridtools/gridtools-base:gcc...	3m 26s
Build GHEX and run unit tests (cpu-clang-ucx-atlas-parmetis, ghcr.io/gridtools/gridtools-base:gcc...	3m 34s
Build GHEX and run unit tests (gpu-hip-mpi-atlas-parmetis, ghcr.io/gridtools/gridtools-base:gcc-9...	5m 42s
Build GHEX and run unit tests (gpu-cuda-mpi-atlas-parmetis, ghcr.io/gridtools/gridtools-base:gcc-...	6m 3s
	24m 55s

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